Primary cilia are solitary appendages that are present on nearly all mammalian cells. Although primary cilia were once considered vestigial, they are now recognized as important cellular sensory and signaling organelles. Defects in the formation or function of primary cilia have been implicated in the pathogenesis of many human developmental disorders and diseases. Yet, the functions of cilia on most cells are unknown. Studies of cilia dysfunction and disease have revealed that the functions of primary cilia are defined by the specific signaling proteins that localize to the membrane of the cilium. These studies have also shown that failure of ciliary signaling proteins to properly localize can lead to disease. However, few ciliary signaling proteins have been identified and the mechanisms that regulate their localization to cilia remain unknown. We have discovered that the genetic defects associated with the human ciliary disorder Bardet-Biedl syndrome (BBS) appear to affect cilia function through a mechanism that disrupts trafficking of G protein-coupled receptors (GPCRs) onto the cilium. Importantly, this finding may represent the fundamental mechanism underlying the pathophysiology of the seemingly diverse BBS phenotypes, including obesity, cognitive deficits, renal cystic disease, and retinal degeneration. The central hypotheses of this proposal are; 1) Ciliary GPCRs contain unique sequences that mediate their localization to cilia, 2) Ciliary localization of GPCRs requires interactions with the BBS proteins, and 3) Mislocalization of ciliary receptors in BBS alters signaling and leads to disease The objectives of this application are to define the intramolecular determinants of GPCR ciliary localization and use this information to identify novel ciliary signaling pathways, define the intermolecular determinants of GPCR ciliary localization and the mechanism(s) by which BBS proteins regulate this process, and determine the effects of ciliary GPCR mislocalization on signaling. These studies will lend valuable insight into the trafficking of ciliary GPCRs and will be used to predict novel ciliary GPCRs. Understanding the determinants and mechanisms of ciliary localization and the effects of a lack of ciliary localization on signaling is essential to elucidating the functions of these organelles and determining their roles in development and disease. Although it is known that almost every cell in the human body possesses an important sensory and signaling appendage called a primary cilium, the functions of cilia on most cells are unknown. The importance of these organelles is highlighted by the fact that defects in primary cilia have been associated with developmental disorders and diseases, including; obesity, renal disease, blindness, nervous system abnormalities, mental retardation, liver disease, and limb defects. The results obtained from the proposed studies will provide important insights into the mechanisms that control the localization of specific signaling proteins to cilia and how disruptions in these mechanisms affect signaling.